Difference amplifier



Feb. 5, 1957 G. KLEIN 2,780,682

DIFFERENCE AMPLIFIER Filed Jan. 24, 1955 INVENTOR GERRIT KLE'IN AGENT ilnited States Patent 6 80,632 DIFFERENCE AMPLIFIER Gerrit Klein, Emmasingel, Eindhoven, Netherlands, as-

signor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee The present invention relates to difference amplifiers. More particularly, the invention relates to difference amplifier circuits utilizing electron discharge tubes in push-pull arrangement.

With various measurements carried out, for example by means of bridge circuits, with examinations by means of electro-cardiographs and with other physiological examinations, it is often required to determine small voltage differences between two points having a comparatively high, variable potential to ground, which potentials are of no importance for the measurements. In order to amplify the voltage to be measured, use may be made of a known difference amplifier, the arrangement of which may be that of Fig. l of the drawing.

An object of the invention is the provision of a difference amplifier utilizing triodes for the amplification which obtains a high rejection factor nevertheless.

in accordance with the present invention, in an ampliher of the type described, each push-pull stage includes a plurality of controlled electron discharge tubes connected in series. Theinput voltages of the tubes are supplied to the control grids thereof, the electrodes of the tubes are at the lowest potential, and the output voltage is obtained between the anodes of the corresponding tubes, which are at a higher potential.

In general, it is SllfllClBIlt to utilize two series connected triodes in each pushfpull stage. The triodes constitute a so-called Oascode, i. e., a cascade connection of a triode in grounded cathode circuit and a triode in grounded grid circuit. This connection operates as a pen tode amplifier, without the occurrence of distribution noise. The grids of the corresponding triodes of the two branches, of which the electrodes are at the highest direct current potential, are connected to each other and are connected through a constant element to the cathode end of a common cathode resistor.

The constant voltage element preferably comprises a gas discharge tube, such as, for example, a glow discharge tube or neon tube, the stabilizing properties of which are known in the art. The electrode of the glow discharge tube not connected to the cathode end of the common resistor is connected through a supply resistor of high resistance value to a point of positive potential. The supply resistor conveys only the discharge current of the glow discharge tube, which may be of very low magnitude. Thus the supply resistor may have a resistance value of, for example, one megohm. This is important since the supply resistor contributes to the determination of the rejection factor. If the supply resistor resistance value is not sufficiently high, said resistor may be replaced by a circuit comprising controlled electron disch'arge tubes having a property of substantially preventing a voltage variation at the negative end from producing a current variation.

The invention will now be more fully described with reference to the accompanying drawing, wherein:

Fig. 1 is a schematic diagram of a diflerence. amplifier circuit of a type known in the art;

Fig. 2 is a schematic diagram of an embodiment of the difference amplifier circuit of the present invention; and

Fig. 3 is a schematic diagram of another embodiment of the difference amplifier circuit of the present inventlon.

In Fig. 1, the input voltages, of which the difference is to be measured, are applied to terminals 1 and 2, which are connected to ground through high resistors 7 and 8 respectively. The terminals 1 and 2 are connected to the control-grids of two amplifying tubes, represented by triodes 5 and 6. The amplified difference voltage is obtained between terminals 3 and 4, which are connected to the anodes of the tubes 5 and 6, respectively. The

anodes are connected through coupling resistors 9 and.

10 to the positive terminal of a source of supply voltage. The cathodes are connected to one another and through a common resistor 11 to the negative'terminal of the voltage supply source, which may have a comparatively high negative potential to ground. The cathode leads may, moreover, include impedances, which are not common to the tubes 5 and 6.

When considering the voltages occurring at the input terminals 1 and 2, it may be advantageous to distinguish between identical components having the same phase (equal-phase voltages) and components being equal in absolute value and having different polarities (unequalphase voltages). The amplifier must be arranged in a manner such that only the unequal-phase voltages are amplified and the equal-phase voltages do not affect the output voltages. With the amplifier shown this is obtained to a certain extent by the provision of the common resistor it. If equal-phase voltages are applied to the control-grids, the tubes 5 and 5 operate in parallel and the resistor 11 produces negative feed-back. In the case of unequal-phase voltages the current variations in the tubes approximately compensate one another in the resistor 11, so that the interconnected cathodes do not substantially change their potentials and the tubes have the normal amplifying effect. If the tubes 5 and 6 are completely identical with one another, the compensation is perfect for equal-phase voltages, so that, if the other stages are also identical and if useis made of a recording member responding only to voltage diiferences, only voltage differences between the two input terminals will be recorded. However, if the tubes 5 and 6 are not identical, equal-phase voltages at the grids will, in general, give rise to unequal-phase voltages between the anodes. This is also the case, if the resistor 11 has a very high resistance value. It has been suggested that the resistor 11 be constituted by a pentode having a resistor in the cathode lead, this resistor being also included in the grid circuit. in this case, as is known, the alternating-current resistance, which is essential with these amplifiers, may be rendered very high. The term alternating-current resistance is to be understood to mean herein the ratio between the voltage variation at the anode of the pentode and a variation in anode current.

As a measure for the quality of a difference amplifier use may in the first place be made of the so-called rejection factor (H). This factor may be defined as the ratio between an equal-phase voltage at the input, producing a given unequal-phase voltage at the output, and the value of an unequal-phase voltage at the input, producing the same unequal-phase voltage at the output. It may be inferred that for an infinite value of the alternating current resistance of the element 11, H is given by:

wherein ,u. designates the mean amplification factor of the two tubes and Au the difference between the two amplification factors.

In order to obtain a high rejection factor, identical tubes could, of course, be selected and utilized in all stages. In this case H would become infinite. As an alternative, the relative difference could be compensated to a certain extent by choosing the static voltages at the electrodes to be different. However, this is circuitous and therefore a difference of about between the various tube values must be taken into account. If use is made of triodes a rejection factor of about 1600 is obtained in this case, which, for practical purposes, is frequently too low. In general, with a maximum difference of 10% in the corresponding tube magnitudes, the minimum rejection factor may be expressed by the formula:

I1 SZ (2) wherein S designates the mean mutual conductance of the tubes, Z the total impedance between the common point of the pushpull connection and ground and is the mean amplification factor from the input grid to the output anode. Where reference is made herein to a high rejection factor, this is to be understood to mean the possible minimum rejection factor.

An improvement may be obtained by using pentodes connected in a particular manner. They should not be used, however, if it is required that there be very low noise.

The arrangement of Fig. 2 is distinguished from that of Fig. 1 mainly in that each of the triodes 5 and 6 is replaced by the series combination of two triodes 5, 12 and 6, 13 respectively. Each series combination constitutes a cascode, since the input voltages are applied to the grids of the triodes 5 and 6 and the interconnected grids of the triodes 12 and 13 have a constant voltage relative to the interconnected cathodes of the triodes 5 and 6. This constant voltage is obtained by connecting the grids of the diodes 12 and 13 through a neon glow discharge tube 14 to the said cathodes. The glow discharge tube 14 is fed through a resistor 15. The resistor 15 may have a very high value, since it is only traversed by the discharge current of the glow discharge tube, which may be about 0.03 milliampere.

The tube pairs 5, 12 and 5, 13 are shown separately, but

they may preferably be united to form a single double triode. As an alternative, the tubes 12, 13 and 5, 6 may be united to form double triodes.

It may be inferred that at an infinite value for the impedance between the cathodes of the tubes 5 and 6 and ground the rejection factor of the arrangement of Fig. 2 is determined by:

For a finite value of the impedance the Formula 2 is applied. The magnitude [.Lla, however, is much higher than with the use of a single tube. Herein pa and u. designate the mean amplification factors of the tubes 5 and 6 and 12 and 13 respectively and a and Ant designate the relative dilferences between the amplification factors of these tube pairs.

From this formula it follows that a great improvement is obtained with respect to the arrangement comprising single triodes. If it is assumed that the permissible deviations from the amplification factors may be 10% in the same directions, which is the most unfavorable case, a rejection factor H for the arrangement of Fig. 2 is found which is /2/Lz times that of the arrangement of Fig. 1. Consequently, it is advantageous to use a high amplification factor, of for example 60, for the triodes, of which the electrodes are at the highest potential.

The requirement that the total impedance between the cathodes and ground be high, may be fulfilled by including in the cathode a circuit exerting a control in a sense such that the current is kept constant; Fig. 3 shows an example thereof. The constant current circuit of Fig. 3

comprises series-connected triodes 16 and 17, the controlgrids of which are connected to points of a potentiometer 118, 19, 20, which may be constituted partly by voltage stabilizers, for example glow discharge tubes. The resistor 11 may have a lower resistance value in this case than in the arrangements of Figs. 1 and 2. The potential difference between the common cathodes and the negative terminal of the supply is much smaller in the arrangement of Fig. 3 than in that of Fig. 2, the alternat ing-current resistance being the same. The arrangement of Fig. 3 is otherwise not distinguished from that of Fig. 2.

The resistor 15 may also be constituted by a circuit comprising controlled tubes, the arrangement being such that a variation in the voltage at the grids of the tubes 12 and 13 does not substantially produce a variation in the current through the resistor 15. However, the requirements for the control-circuit replacing the resistor 15' are less severe than for the resistor 11, since the current strength is much lower. The amplifiers according to the invention described above are, moreover, particularly suitable for direct-voltage amplification.

While the invention has been described by means of specific examples and in specific embodiments, I do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. A difference amplifier comprising a push-pull connected amplifying system, each branch of said push-pull system including a plurality of series-connected electron discharge tubes having control and anode electrodes, means for applying a potential across said series-connected discharge tubes, means for applying an input voltage to the control electrode of a first tube of said plurality of tubes of each of said branches having the lowest potential applied thereto, and means for deriving an output voltage between the anodes of a second tube of said plurality of tubes of each branch having a higher potential applied thereto than said lowest potential, an impedance element for providing a constant voltage thereacross, and a common resistor connected at one end to the cathode of each of said first tubes, the control electrode of each of said second tubes being connected to the cathode of each of said first tubes through said impedance element, said means for applying a potential across said tubes including a negative terminal connected to the cathode of each of said first tubes through said common resistor.

2. A difierence amplifier comprising a push-pull connected amplifying system, each branch of said push-pull system including a plurality of series-connected electron discharge tubes having control and anode electrodes, means for applying a potential across said series-connected discharge tubes, means for applying an input voltage to the control electrode of a first tube of said plurality of tubes of each of said branches having the lowest potential applied thereto, and means for deriving an output voltage between the anodes of a second tube of said plu rality of tubes of each branch having a higher potential applied thereto than said lowest potential, a glow discharge tube for providing a constant voltage thereacross, and a common resistor connected at one end to the oathode of each of said first tubes, the control electrode of each of said second tubes being connected to the cathode of each of said first tubes through said glow discharge tube, said means for applying a potential across said tubes including a negative terminal connected to the cathode of each of said first tubes through said common resistor.

3. A diiferenw amplifier comprising a push-pull connected amplifying system, each branch of said push-pull system including a plurality of series-connected electron discharge tubes having control and anode electrodes, means for applying a potential across said series-connected discharge tubes, means for applying an input voltage to the control electrode of a first tube of said plurality of tubes of each of said branches having the lowest p0- tential applied thereto, and means for deriving an output voltage between the anodes of a second tube of said plurality of tubes of each branch having a higher potential applied thereto than said lowest potential, an impedance element for providing a constant voltage thereacross, and a common resistor connected at one end to the cathode of each of said first tubes comprising a potentiometer and two series-connected triodes having control grids connected to points on said potentiometer, the control electrode of each of said second tubes being connected to 15? the cathode of each of said first tubes through said im- 6 pedance element, said means for applying a potential across said tubes including a negative terminal connected to the cathode of each of said first tubes through said common resistor.

References Cited in the file of this patent UNITED STATES PATENTS 2,329,073 Mitchell et al Sept. 7, 1943 2,424,893 Mansford July 29, 1947 2,592,193 Saunders Apr. 8, 1952 2,631,198 P-arisoe Mar. 10, 1953 

